Rotary pump inlet velocity profile control device

ABSTRACT

A rotary pump with an inlet flow duct having a convergent section upstream the tips of the rotor blades. The convergent section decreases the cross-sectional flow area of the inlet flow duct prior to the flow being introduced into the rotor, thereby creating a substantially uniform velocity profile in the flow just upstream the rotor blades.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements in rotary pumps, andparticularly to increasing the performance of rotary pumps by modifyingthe velocity profile upstream of the rotor.

2. Description of the Prior Art

The design procedure for most prior art rotary pumps is based on theassumption of a uniform pump inlet velocity from rotor hub to tip.Unfortunately however, the inlet velocity profile in conventional rotarypumps is not uniform. A non-uniform pump inlet velocity results, inpart, from the boundary layer and in part from the cascade inducedincidence (CII) effect angle. (See, for example, Scholz, Norbert,"Aerodynamics of Cascades", an English revised version AGARD 1977, pg.211.)

The typically designed inducer leading edge hub-tip blade angledistribution may be represented by the equation:

R·tan β=constant, where

R=radius at a location between the hub and the tip

β=blade angle corresponding to R

In actual non-uniform flow, when a blade is constructed in accordancewith the above equation, the tip will experience a higher incidenceangle than predicted. The hub will have a much lower incidence anglethan predicted. Therefore, conventional design procedures result inreduced pump suction capability and pump efficiency.

OBJECTS AND SUMMARY OF THE INVENTION

The principal object of the present invention therefore is to provide arotary pump which is highly efficient and low in cost.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawing.

These objects are achieved by providing an inlet duct to the rotary pumpwhich is convergent just upstream the rotor blades. The convergent inletflow duct has a geometry defined by the relationship: ##EQU1## n=2 whereRe≦2300 n=2+0.00432 (Re-2300) where 2300<Re<3200, and

n=3Re^(1/12) where Re≧3200

σ=R_(HUB) /R_(T), and

0.8≦K≦1

By utilizing a convergent inlet duct as defined in the aboverelationship, the boundary layer flow and the unique geometry (R·tanβ=constant) of the rotor including the rotor blades is compensated for.The convergent duct results in fluid having a substantially uniformvelocity profile being introduced into the rotor blades.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view, in partial cross section, of apreferred embodiment of the present invention showing aninducer/impeller rotary pump.

FIG. 2 is an end view of the rotary pump taken along line 2--2 of FIG.1.

FIG. 3 is a graph which illustrates the velocity distribution inconvergent and divergent channels with flat walls.

FIG. 4 is a graph which illustrates pressure looses within a contractionpipe.

FIG. 5 shows a model of a rotary pump embodying the principles of thepresent invention useful for theoretical consideration.

FIG. 6 is a schematic side view, in partial cross section, of apreferred embodiment of the present invention including a rotary pumphaving an inducer.

FIG. 7 is a schematic view, in partial cross section, of a preferredembodiment of the present invention including a rotary pump having animpeller.

The same elements or parts throughout the figures are designated by thesame reference characters.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a preferred embodiment of the present invention isdepicted comprising elements of a rotary pump 10 constructed inaccordance with the present invention. The pump includes a housing 12containing a rotatable rotor generally designated 14 provided with ashaft 16 and impeller 18.

The rotor 14 has an upstream end with a hub surface 20 of revolutionthereon. A plurality of rotor blades 22 extend radially from hub surface20. The portion of the rotary pump 10 which contains hub surface 20 andblades 22 is commonly referred to as the inducer. However, as explainedbelow with relation to other embodiments a rotary pump embodying theprinciples of the present invention does not necessarily require aninducer. Thus, to prevent any ambiguities in the claim language below,the inducer blades are described herein generally as rotor blades. Eachrotor blade 22 has a leading edge 24. The blades 22 are axially aligned.Thus, a circle 26 with a radius R_(HUB) is formed, defined by theintersection of each leading edge 24 with the hub surface 20. (See FIG.2) (R_(HUB) is known in the art as the leading edge hub radius.) Eachrotor blade 22 terminates in a tip 28. The tips 28 define a secondcircle 30 having a radius R_(TIP).

The inlet flow duct to the rotary pump 10 is designated generally as 32.A first section, labeled A, upstream the rotor 14 has a substantiallyconstant radius R_(o). A second section, B, downstream the firstsection, A, but upstream the blade tip 28 is convergent. A thirdsection, C, downstream the second section has a radius R_(T) which isslightly larger than R_(TIP) (i.e. sufficient to provide clearance forthe tips 28). The flow duct 32 has a geometry defined by therelationship, ##EQU2## n=2 where Re≦2300 n=2+0.00432 (Re-2300) where2300<Re<3200, and

n=3Re^(1/12) where Re≧3200

σ=R_(HUB) /R_(T), and

0.8≦K≦1

Equation 1 is derived from the following theoretical considerations:

The literature demonstrates that the boundary layer in a convergent ductis much thinner than a divergent or constant area duct. FIG. 3 is agraph excerpted from Schlichting, H., "Boundary-Layer Theory", 1979,published by McGraw-Hill, Inc., pg. 669. The graph illustrates thevelocity distribution in convergent ducts, divergent ducts and constantarea ducts.

The abscissa corresponds to the locations from the center of the duct indimensionless units, where:

y=distance from the center of the duct

B=diameter of the duct

The ordinate corresponds to velocity in dimensionless units, where:

n=local velocity

U=maximum velocity (i.e. at the center of the pipe)

The curves represent the velocity distribution for ducts with half-cone(included) angles, α between -8° and 4°, where the negative signrepresents a convergent duct. As can be seen from the illustration, theboundary layer becomes very thin with convergent ducts. Therefore, if aconvergent duct is utilized just upstream the rotor blades, the inletvelocity distribution will be substantially uniform and the leading edgeblade angle distribution from hub to tip, R·tan β, will be accurate. TheR·tan β blade designed for a uniform velocity distribution is simple todescribe and easier to fabricate than the complex shapes required tomatch a non-uniform velocity profile. Without a convergent inlet, therotor leading edge blade, in order to optimize performance, would haveto be complicated and difficult to fabricate.

A question regarding possible extra losses by the use of a convergentpipe may be raised. However, further reference to the literatureindicates that the losses would be relatively small for a convergentduct. The graph of FIG. 4 illustrates the pressure losses given themodel designated 34 in that Figure. (This Figure is excerpted fromS.A.E. Aerospace Applied Thermodynamics Manual, Second Edition, 1969,page 19.) Although FIG. 4 assumes a pipe converging by a radius R, themodel provides an approximation as to the worst possible pressure lossthat might result from the convergence of the subject inlet duct. Forapplicants' anticipated purposes, the subject inlet duct has a ratio ofr/d₂ <0.12, thus K_(t) is less than 3% of the exit velocity head. Thispressure loss is more than compensated for by the benefits of thematched design.

A schematic illustration of a convergent duct 36 in front of a rotor 38is shown in FIG. 5. In view of the above discussion, it is assumed thatthe total pressure losses due to the duct contraction are minimal (i.e.applicant's inlet duct would have a curvature which is less than theabruptness created by a radius of a circle, which was the assumptionmade above relating to FIG. 4).

Assuming that the velocity is constant at Section B (i.e. the boundarylayer is negligible), then

    U.sub.B =Q/A                                               (2)

where, Q=flow rate, U_(B) =blade leading edge velocity, and

    A=π(R.sub.T.sup.2 -R.sub.HUB.sup.2)                     (3) ##EQU3##

If σ=R_(HUB) /R_(TIP) is substituted into Equation 4; then ##EQU4##

(It is assumed that for the purposes of this equation T_(T) ≃R_(TIP).)

The fully developed pipe flow profile, as defined in Schlichting, H.,"Boundary-Layer Theory", 1979, by McGraw-Hill, Inc., pg. 559 is:##EQU5## where: n=2 where Re≦2300,

n=2+0.00432 (Re-2300) where 2300<Re<3200, and

n=3Re^(1/12) where Re>3200;

U_(A) =average velocity at section A; and

U_(MAX) =KU.

Studies by applicants conclude that 0.8≦K≦1 allows attainment of areasonably uniform inducer leading edge profile.

Solving Equations 5 and 6 for R_(o) results in the followingrelationship: ##EQU6##

In some instances the rotor may actually protrude into Section A asshown by phantom lines 40. Conservative design practices would includesuch a presumption. Therefore, the resulting workable equation is thatlabeled above as Equation 1.

Utilizing a convergent inlet duct provides an expedient manner ofmodifying the velocity profile upstream of the blade tips into a uniformflow thereby allowing a simple rotor blade hub-to-tip blade angledistribution to match the flow. The simple blading reduces rotorfabrication cost. The better flow match improves pump suctionperformance and pump operating life. Studies by applicants demonstratethat suction capability improves up to 20% and efficiency up to 5% byutilization of the subject inlet duct.

Referring back to FIG. 1, in operation, torque is applied to rotor 14from an external power source (not shown). A fluid is introduced throughthe convergent section B of inlet duct 32. The velocity profile is madesubstantially uniform by decreasing the boundary layer. The flow thenproceeds between the inducer blade 22 of the inducer and then throughthe impeller 18. The flow is then discharged radially through an exitduct 42.

As noted above it is to be understood that this invention is not limitedto the inducer/impeller, combination of the above described embodiment,although such an arrangement is desirable for high suction performanceand high discharge pressure applications.

FIG. 6 illustrates a rotary pump 44 which includes a rotor/inducergenerally designated 46 and is absent the impeller found in the previousembodiment. The embodiment of FIG. 6 is desirable for high suctionperformance and low discharge pressure applications. Fluid flows throughthe convergent inlet duct 48 which produces a uniform velocity profilein the fluid therein. The fluid then flows through the inducer/rotorblades 50 and finally exits axially through the exit duct 52.

FIG. 7 illustrates a rotary pump 52 which includes a rotor/impeller 54and is absent the inducer found in either of the previous embodiments.The embodiment of FIG. 7 is desirable for high discharge pressure/lowsuction performance applications. Fluid flows through the convergentinlet duct 56 through the impeller blades 58 and radially out the exitduct 60.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

For example, in FIGS. 1, 6 and 7, the convergency in the inlet duct isshown to be linearly tapered. However, the duct may be smoothly curvedin various fashions as long as the R_(o) is as prescribed in the aboveequations in order to provide a substantially constant velocity profile.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A rotary pump, comprising:a housing; a rotorrotatably attached to said housing having an upstream end with a hubsurface of revolution thereon; a plurality of rotor blades extendingradially from said hub surface, each rotor blade having a leading edge,each leading edge intersecting said hub surface at substantially thesame axial position, said intersections defining a first circle with aradius being the leading edge hub radius, R_(HUB), each rotor bladeterminating in a tip, said tips defining a second circle having aradius, R_(TIP) ; and a flow duct attached to said housing forintroducing flow into said rotor, said flow duct having a first sectionupstream said rotor with a substantially constant radius, R_(o), aconvergent second section downstream said first section but upstreamsaid tips, and a third section downstream said second section having aradius R_(T) being approximately equal to R_(TIP), said flow duct havinga geometry defined by the relationship, ##EQU7## n=2 where Re≦2300,n=2+0.00432 (Re-2300) where 2300<Re<3200, and n=3Re^(1/12) whereRe>3200; σ=R_(HUB) /R_(T) ; and,
 0. 8≦K≦1.
 2. A rotary pump,comprising:a housing; a rotor rotatably attached to said housing havingan upstream end with a hub surface of revolution thereon; a plurality ofrotor blades extending radially from said hub surface, each rotor bladehaving a leading edge and each terminating in a tip, each leading edgeintersects said hub surface at substantially the same axial position,said intersection defining a first circle with a radius being theleading edge hub radius, R_(HUB), said tips defining a second circlehaving a radius, R_(TIP) ; and inlet flow duct means attached to saidhousing for introducing flow into said rotor, said inlet flow duct meanshaving a convergence section upstream said tips with a substantiallyconstant radius, R_(O), a convergence section downstream said firstsection but upstream said tips, and a third section downstream saidsecond section having a radius, R_(t) being approximately equal toR_(TIP), said flow duct having a geometry defined by the relationship,##EQU8## n=2 where Re≦2300, n=2+0.00432 (Re-2300) where 2300<Re<3200,and n+3Re^(1/12) where Re>3200; σ=R_(HUB) /R_(T) ; and,
 0. 8≦K≦1.